According to many women, a heightened sense of smell, called hyperosmia, is one of their earliest signs of pregnancy.
Although anecdotes of hyperosmia have existed for a century, scientific literature on the topic is sparse. Subjectively, around two-thirds of women rate their sense of smell as stronger than usual while pregnant. Another study reported that, compared with women who aren’t pregnant, pregnant women are particularly sensitive to odours such as cooking foods, cigarette smoke, spoiled food, perfumes, and spices.
A few studies have examined scent detection thresholds (the smallest volume of air that still results in odour detection) in pregnant versus non-pregnant women. But, in a study where six different scents were tested, there was no difference in detection threshold between the two groups.
Given the inconsistency between subjective and objective reports on hyperosmia, research suggests that pregnant women don’t necessarily have a stronger sense of smell, but are perhaps better at identifying smells. A recent study found that pregnant women were more likely to rate a variety of smells as significantly less pleasant than non-pregnant women. Early in pregnancy —much like the body rejects foods that may be toxic to the developing foetus — it appears that women have a greater “disgust sensitivity” that motivates them to avoid other possible contaminants (why cigarettes smoke and spoiled food may seem particularly pungent).
As with morning sickness, there’s a link between the timing of hCG levels and when a women’s odour perception changes. But it’s thought that these hormonal changes aren’t affecting our noses. When a Swedish research group presented scents to pregnant and non-pregnant women and measured their brains’ responses, they found a larger amplitude and shorter latency un the P300 wave of pregnant women, a voltage change (thought to reflect neural processes) related to a person’s assessment and evaluation of an event. This suggests that hormonal changes may be acting on higher-order cognitive processes related to our perception of odours.
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